This application claims priority from Korean Patent Application No. 2002-71390, filed on 16 Nov. 2002, in the Korean Intellectual Property Office, the contents of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a super twist nematic (STN) liquid crystal display (LCD) driver, and more particularly, to a driver for driving a STN LCD panel by using a multi-line selection method and an Alt-Pleshko technique (APT).
2. Description of the Related Art
In a super twist nematic (STN) liquid crystal display (LCD), a character or a picture is displayed on a screen by a liquid crystalline polymer twisted to reflect light. The liquid crystalline polymer is called nematic. The STN LCD is used to display simple numbers or characters in a calculator, a display unit of a mobile phone, and so on.
The STN LCD presents colors through contrast in gray scale. A screen using 16 shades of gray can present colors through 16 different contrasts, thus there is no problem in presenting text files inputted, outputted, or edited in the screen. The STN LCD basically uses white and black inversion and can improve visibility by presenting black characteristics on a white background or white characteristics on a black background in a text mode.
The method of driving the STN LCD includes a multi line selection method and a single line selection method. If the response time of the STN LCD is increased, a so-called frame response occurs causing flickering or a deterioration of contrast.
The multi-line selection method is used to solve such problems. The multi-line selection method selects a plurality of scan electrode lines at one time.
The multi-line selection method includes a three-line selection method and a four-line selection method.
FIG. 1 is a circuit diagram of an STN LCD driver 100 using the four-line selection method.
The STN LCD driver 100 requires at least seven voltage levels to drive a STN LCD panel (not shown).
A first common voltage V3 and a second common voltage −MV3 are applied to a row line of the STN LCD panel (not shown). The number of transitions of the first common voltage V3 and the second common voltage −MV3, i.e., voltages applied to the row line, is smaller than that of first through fifth segment voltages VC, V1, V2, MV1, and MV2 which are applied to a column line. Thus, the width of the voltage swing of the voltages applied to the row line becomes great, which means that the voltage level applied to the row line is high.
The first through fifth segment voltages VC, V1, V2, MV1, and MV2 are applied to the column line of the STN LCD panel (not shown) as described above. The first segment voltage VC, the second segment voltage V1, the third segment voltage V2, the fourth segment voltage MV1, and the fifth segment voltage MV2 are generated from a reference voltage generating unit 130 and a voltage control circuit 140. The relationship between the voltage levels of the first through fifth segment voltages VC, V1, V2, MV1, and MV2 is V2>V1>VC>MV1>MV2. The differences between two voltage levels, for example, V2 and V1, or V1 and VC, are equal to one another.
A supply voltage VDD is applied to the reference voltage generating unit 130 and a first boosting circuit 150. The reference voltage generating unit 130, which receives the supply voltage VDD, generates the third segment voltage V2 by using the voltage control circuit 140. Voltage levels between the third segment voltage V2 and a ground voltage GND are divided by resistances R into the first segment voltage VC, the second segment voltage V1, and the fourth segment voltage MV1. A boosting voltage VCSL outputted from the first boosting circuit 150 is applied to voltage followers 160, 170, and 180. The voltage followers 170, 160, and 180 stabilize the first segment voltage VC, the second segment voltage V1, and the fourth segment voltage MV1, respectively.
The first common voltage V3 is generated from a second boosting circuit 110. The second boosting circuit 110 receives the third segment voltage V2, boosts the level of the third segment voltage V2 to twice the level of the third segment voltage V2, and outputs the first common voltage V3.
A second common voltage −MV3 is generated from the second dropping circuit 120. The second dropping circuit 120 receives the third segment voltage V2, boosts the level of the third segment voltage V2 to twice the level of the ground voltage GND, and outputs the second common voltage −MV3.
FIG. 2 is a circuit diagram of an STN LCD driver 200 using a three-line selection method.
The STN LCD driver 200 using the three-line selection method requires at least five voltage levels to drive a STN LCD panel (not shown).
A first common voltage +VR and a second common voltage −VR are applied to a row line of the STN LCD panel (not shown). The number of transitions of the first common voltage +VR and the second common voltage −VR, i.e., the voltages applied to the row line, is smaller than voltages V1, VM, and GND applied to a column of the STN LCD panel. Thus, the width of the voltage swing of the voltages applied to the row line becomes great.
First through third segment voltages VM, V1, and VSS are applied to the column line of the STN LCD panel (not shown). The relationship between voltage levels of the first through third segment voltage VM, V1, and VSS is V1>VM>VSS. The differences between two voltage levels, e.g., V1 and VM, or VM and VSS, are equal to one another.
An external voltage VCI is applied to a reference voltage generating unit 230 and a first boosting circuit 260 differently from the STN CLD driver 100 using the four-line selection method of FIG. 1. The reference voltage generating unit 230 which receives the external voltage VCI generates the second segment voltage V1 by using a voltage control circuit 240 and an electric volume 250. The voltage levels between the second segment voltage V1 and the third segment voltage VSS are divided by resistances R and an adjustable resistance RV into the first segment voltage VM.
A random voltage VX is required to generate the second common voltage −VR. The level of the random voltage VX is determined by the adjustable resistance RV. The second common voltage −VR is generated by boosting the random voltage VX to three through five times its original value.
The first common voltage +VR is generated by boosting the second common voltage −VR to twice the second common voltage −VR.
Both the STN LCD drivers 100 and 200 of FIGS. 1 and 2 require a boosting circuit and a dropping circuit to generate a common voltage at a high level. The boosting circuit and the dropping circuit require an external capacitor to boost and drop voltages.
A bias capacitor is required to avoid the effect of loads of the STN LCD panel to the segment voltages and common voltages. An external capacitor is presented as C in FIG. 2, and a bias capacitor is presented as C1 in FIG. 2.
Referring to FIG. 1, the first boosting circuit 150 requires at least two external capacitors to boost the supply voltage VDD to twice the supply voltage VDD and at least three external capacitors to boost the supply voltage VDD to three times the supply voltage VDD. The second boosting circuit 110 requires at least two external capacitors to boost and drop voltages to twice the voltages. The second dropping circuit 120 requires at least two external capacitors to boost and drop voltages to twice the voltages. At least four bias capacitors are required to stabilize the first through fourth segment voltages VC, V1, V2, and MV1. Thus, the STN LCD driver 100 requires at least 11 total capacitors.
Referring to FIG. 2, the first boosting circuit 260 requires at least two external capacitors to boost the external voltage VCI to twice the external voltage VCI, and the second boosting circuit 210 requires at least two external capacitors to boost voltages to twice the voltages. The second dropping circuit 220 requires at least three through five external capacitors to boost the random voltage VX to three through five times the random voltage VX. At least three bias capacitors are required to stabilize the first segment voltage VM, the second segment voltage V1, and the random voltage VX. Thus, the STN LCD driver 200 requires 10–12 capacitors.
When a number of external capacitors are included in an STN LCD driver, module manufacturers have to bear increases in the production cost and the number of defective modules, and the volume of the module increases as well. In addition, since the boosting circuit and the dropping circuit have to operate continuously, power consumption increases accordingly.